1. Field of Invention
The invention relates to the field of metal forming and, in particular, to an improvement to the process of superplastic forming of structures wherein a portion of a heated preform is expanded against the surface of forming member. Specifically, it relates to a method of selectively applying coatings to the forming member or the preform, which produce different coefficients of friction between portions of the preform and the forming member during expansion, providing a more uniform thickness profile in the structure thus formed.
2. Description of the Prior Art
Superplasticity is the capability of a material to develop unusually high tensile elongation with a reduced tendency toward local necking during deformation at elevated temperatures. Alloys which exhibit superplasticity are capable of being subjected to superplastic forming wherein portions of a preform are expanded, by the application of fluid pressure, against the surface of a forming member, usually in the form of a die, to produce structures of predetermined shapes. Such expansion is in the form of an increase in the surface area produced by an elongation in the length and a reduction in thickness of individual material elements. The process has been recently combined with diffusion bonding to produce structures typically costing 50% less to fabricate, while often achieving a 30% weight reduction over conventional fabrication techniques.
But many of these metals are subject to surface contamination at forming temperatures. Such metals are termed "reactive" metals, and include alloys of titanium and zirconium. Therefore, during forming at high temperatures the preform must be protected by an inert release coating so that it does not react with the forming member itself. A release coating typically is formulated to prevent sticking of the preform to the forming member. In superplastic forming its primary purpose is to act as a releasing agent so as to facilitate the removal of the expanded preform from the forming member. This coating will also affect the sliding friction of the preform in the forming member during expansion. There are several commercially available release coatings which are suitable which usually contain compounds of alumina, graphite, boron nitride or yttria.
Expanding a preform in a superplastic condition differs from the more conventional expanding processes. For example, U.S. Pat. No. 3,769,834, Deep Drawing Method by Wayne C. Granzow, discloses a method of room temperature drawing of aluminum wherein a flat preform is clamped at its periphery over a die cavity. A punch is used to drive the preform into the cavity, drawing in metal from under the clamped portion.
In superplastic forming operations, which is disclosed in U.S. Pat. No. 3,934,441, Controlled Environment Superplastic Forming of Metals by C. H. Hamilton, et al., the preform is clamped firmly at its periphery, thus ideally allowing for material to be drawn from this area. However, due to a combination of the compressive forces from the forming member and tensile forces from the fluid pressure applied to the expanding portions of the preform, some expanding in the form of elongation or stretching occurs at the periphery. Such elongation or stretching also occurs in the forming member cavity entrance radius area, i.e., the intermediate region between the peripheral portion of the preform and the part expanding into the cavity. As the preform drapes over the radius area there is a tendency to increase the rate of material elongation which, in turn, may produce local necking in this area. This makes it difficult to obtain uniform thickness profiles in the structure. For example, when fabricating a simple hat section, if a release coating which is capable of producing a high coefficient of friction is used, such as one containing yttria, there is an increase in the frictional force and a lower net force causing material expansion. Thus there is a reduction in the expansion itself. The result is that the preform remains thick at the radius area, while, the portion expanded within the cavity will have an uneven thickness profile. If a release coating which produces a relatively low coefficient of friction is used, such as one containing boron nitride, there is a more uniform thickness profile obtained in the portion expanded into the cavity, but pronounced local thinning near the radius area.
Accordingly, one object of this invention is to provide an improved superplastic forming process.
Another object of this invention is to provide a process which produces a more uniform thinning profile of a structure produced by superplastic forming.
A further object of this invention is to provide a coating process for use in superplastic forming which provides both a more uniform thinning profile of the structure thus formed, while reducing surface contamination and facilitating release of the formed structure from the forming member.